grease pencil keyframe navigation?
In Blender2.79 ,Grease pencil keyframes are shown in the Dopesheet ->Grease pencil Panel.
Hotkeys should be same for other Blender versions as well.

IMPORTANT - Mouse Cursor  should be in the Dopesheet Editor for Blender to recognise the scope of the Hotkey.

For moving inbetween keyframes, following Hotkeys are available 

1.First/Last Timeline Frame - Shift Ctrl UpArrow /Shift Ctrl DownArrow
2.Next Keyframe - UpArrow
3.Previous Keyframe - DownArrow
4.Play/Stop Animation - Alt A/Shift Alt A (Version Blender2.79 and below) &amp;
   Play/Stop Animation - Spacebar (Version Blender2.8 and above)